Method for preparing purified pyrocatechol

Abstract

The subject of the present invention is a method for preparing pyrocatechol from which the impurities resulting from the method for the preparation thereof have been removed. The method for preparing purified pyrocatechol from a crude pyrocatechol containing essentially pyrocatechol, small amounts of impurities including dihydroxybenzoquinone, and traces of hydroquinone and of phenolic compounds, is characterized in that it comprises at least the following steps: dissolution of the crude pyrocatechol in water, crystallization of the pyrocatechol, separation of the purified pyrocatechol and, optionally, a step of drying the purified pyrocatechol. The method of the invention may include other steps and, depending on the embodiment chosen, that may comprise a different series of steps, it is possible to obtain pyrocatechol with various degrees of purity.

The invention claimed is: 
     
       1. A process for preparing purified pyrocatechol from a crude pyrocatechol comprising pyrocatechol, small amounts of impurities including dihydroxybenzoquinone, traces of hydroquinone, and phenolic compounds in a content of less than 5% by mass, comprising at least the following steps:
 dissolving the crude pyrocatechol in water to a concentration ranging from 40% to 90% by mass; 
 crystallizing the pyrocatechol, by cooling to a temperature ranging from 0 to 20° C.; 
 separating the crystalline pyrocatechol from an aqueous phase formed from the crystallization mother liquors; and 
 optionally drying the purified pyrocatechol. 
 
     
     
       2. The process of  claim 1 , wherein the purified pyrocatechol is dried. 
     
     
       3. The process of  claim 2 , wherein the step of crystallizing the pyrocatechol comprises evaporation and outputting a stream of condensates. 
     
     
       4. The process of  claim 2 , further comprising washing the crystallized pyrocatechol with water and optionally collecting the washing liquors. 
     
     
       5. The process of  claim 3 , further comprising washing the crystallized pyrocatechol with water and optionally collecting the washing liquors. 
     
     
       6. The process of  claim 1 , further comprising the steps of:
 recycling a stream comprising the aqueous phase formed from the crystallization mother liquors into the dissolving or crystallizing step; 
 optionally concentrating the stream; and 
 purging the stream, optionally after concentrating said stream. 
 
     
     
       7. The process of  claim 6 , wherein the step of crystallizing the pyrocatechol comprises evaporation and outputting a stream of condensates. 
     
     
       8. The process of  claim 6 , further comprising the step of:
 washing the separated pyrocatechol with water and collecting washing liquors. 
 
     
     
       9. The process of  claim 8 , wherein the washing liquors are mixed with the aqueous phase formed from the crystallization mother liquors before said concentrating. 
     
     
       10. The process of  claim 7 , further comprising the step of:
 washing the separated pyrocatechol with water and collecting washing liquors. 
 
     
     
       11. The process of  claim 10 , wherein the washing liquors are mixed with the aqueous phase formed from the crystallization mother liquors before said concentrating. 
     
     
       12. The process of  claim 1 , wherein said crude pyrocatechol is dissolved in water to a concentration ranging from 50% to 90% by mass. 
     
     
       13. The process of  claim 1 , wherein the dissolving step is performed at a temperature ranging from 40° C. to 100° C. 
     
     
       14. The process of  claim 1 , wherein crystallization the pyrocatechol is performed by cooling from the dissolution temperature to a temperature ranging from 0 to 10° C. 
     
     
       15. The process of  claim 1 , wherein crystallization seeds are introduced in an amount less than 2% by mass relative to the mass of pyrocatechol crystals to be obtained. 
     
     
       16. The process of  claim 2 , wherein the pyrocatechol is dried at a drying temperature ranging from 50° C. to 100° C. 
     
     
       17. The process of  claim 3 , wherein the evaporation is performed before or during the crystallization and comprises heating at atmospheric pressure or under a reduced pressure ranging from 25 mbar to 1 bar, at a temperature ranging from 25° C. to 100° C. 
     
     
       18. The process of  claim 5 , wherein the mass ratio of washing water to mother liquors comprising the moisture content of the pyrocatechol is not more than 3. 
     
     
       19. The process of  claim 6 , comprising concentrating the recycled stream to an amount of pyrocatechol ranging from 10% to 90% by mass. 
     
     
       20. The process of  claim 19 , wherein concentrating the recycled stream comprises heating said stream at atmospheric pressure or under a reduced pressure, at a temperature ranging from 70° C. to 100° C. 
     
     
       21. The process of  claim 1 , further comprising a step of treating the crude pyrocatechol in water with carbon black or active charcoal before the crystallizing step, wherein said carbon black or active charcoal is optionally separated from the pyrocatechol after said treatment. 
     
     
       22. The process of  claim 1 , wherein one or more steps are performed under an atmosphere of inert gas. 
     
     
       23. The process of  claim 1 , wherein the crude pyrocatechol comprises impurities in an amount ranging from 0.1% to 2.5% by mass relative to the total mass of the crude pyrocatechol. 
     
     
       24. The process of  claim 1 , wherein the mass ratio of the dihydroxybenzoquinone to the sum of the impurities ranges from 0.5 to 0.8. 
     
     
       25. The process of  claim 1 , wherein the mass ratio of the phenolic compounds to the sum of the impurities ranges from 0.18 to 0.4. 
     
     
       26. The process of  claim 1 , wherein the mass ratio of the hydroquinone to the sum of the impurities ranges from 0.2 to 0.10. 
     
     
       27. The process of  claim 1 , wherein the crude pyrocatechol comprises, by mass relative to the total amount of crude pyrocatechol:
 from 97.5% to 99.9% of pyrocatechol, 
 from 0.003% to 0.07% of hydroquinone, 
 from 0.02% to 0.5% of phenolic compounds, and 
 from 0.07% to 1.5% of hydroxybenzoquinone. 
 
     
     
       28. The process of  claim 1 , wherein the crude pyrocatechol is obtainable from a hydroxylation reaction of phenol with hydrogen peroxide in the presence of an acid catalyst followed by distillation of said mixture. 
     
     
       29. The process of  claim 28 , wherein said pyrocatechol is obtainable from a distillation head and a mixture consisting essentially of hydroquinone with a small amount of impurities is obtainable from a distillation tail. 
     
     
       30. The process of  claim 1 , wherein said purified pyrocatechol has a purity ranging from 99% to 99.995%. 
     
     
       31. The process of  claim 1 , wherein said purified pyrocatechol has a purity ranging from 99.5% to 99.99%. 
     
     
       32. The process of  claim 30 , wherein a said purified pyrocatechol comprises less than 10 ppm of hydroquinone, 40 ppm of dihydroxybenzoquinone, and 40 ppm of phenolic compounds.